1. Field of the Invention
The present invention relates to an instrument for transmitting laser radiation for application to biological tissue for removal, penetration or treatment of the tissue and more particularly to an instrument for efficiently and accurately delivering laser radiation to a predetermined location on the biological tissue.
2. Description of Related Art
Surgical applications of lasers are well established in opthalmology, otolaryngology, gynecology, dermatology and plastic surgery, having been in use, in some cases, for over two decades. Lasers have also become well accepted in the treatment of cardiovascular diseases. The types of lasers are nearly as numerous as the procedures that use them, and selection of a laser for any given procedure depends upon the laser-tissue interaction, which phenomena have been widely reported, and the desired outcome of that interaction. See, e.g., M. J. C. van Gemert and A. J. Welch, "Time Constants in Thermal Laser Medicine", Lasers in Surgery and Medicine 9:405-421 (1989); and J. L. Ratz, "Laser Physics", Clinics in Dermatology 13:11-20 (1995), which are incorporated herein by reference. The types of lasers may be grouped into ultraviolet (193-351 nm), visible wavelength (400-700 nm), and infrared (700-100,000 nm). The visible light lasers, such as argon (488-514 nm), flashlamp-pumped dye (510 nm), copper vapor (578 nm) and ruby (694 nm), are commonly used for selective photothermalysis, e.g., photocoagulation of vascular and pigmented lesions. Laser light within the visible range can be delivered using a number of conventional optical techniques including refractive lenses and quartz fiber optics. Examples of visible light delivery systems are provided in U.S. Pat. No. 5,207,673 of Ebling, et al., "Fiber Optic Apparatus for Use with Medical Lasers", No. 5,495,541 of Murray, et al., "Optical Delivery Device with High Numerical Aperture Curved Waveguide", and No. 5,530,780 of Ohsawa, "Fiber Optic Laser Conducting and Diffusion Device", the disclosures of which are incorporated herein by reference. Ultraviolet (UV) lasers, or excimer lasers, which include argon-fluoride (193 nm) and krypton-fluoride (248 nm), have been used predominantly in photorefractive keratectomy to ablate corneal tissue. Excimer lasers have also been reported for ablation of skin. (See, e.g., R. J. Lane, et al., "Ultraviolet-Laser Ablation of Skin", Arch. Dermatol.--121: 609-617 (May 1985).)
Visible, UV and near IR laser light have been combined with surgical tips to provide precise control of application of laser radiation and/or to provide means for coagulating blood adjacent an incision. U.S. Pat. No. 4,126,136 of Auth, et al., describes a transparent scalpel blade connected to a fiber optic waveguide which transports laser radiation to the blade. The blade, which is preferably synthetic sapphire (Al.sub.2 O.sub.3), emits laser radiation through the tapered cutting edge to photocoagulate the blood. U.S. Pat. No. 4,627,435 of Hoskin discloses a surgical knife formed from a diamond blade optically coupled to a Nd:YAG laser by a fiber optic bundle. The diamond blade is heated by the laser radiation to provide a cauterizing action while making the incision. The diamond blade may also be coupled to a visible laser to provide illumination for enhanced visibility of the incision site. U.S. Pat. No. 4,693,244 of Daikuzono describes an artificial sapphire tip coupled to a quartz optical fiber to transmit radiation from a Nd:YAG laser. The sapphire tip is heated by the radiation to coagulate the blood at an incision made with a separate surgical blade. U.S. Pat. No. 5,320,620 of Long, et al., describes a laser surgical device with a blunt light emitting element for coagulation. The tip, which may be sapphire, silica or YAG, is coupled to an optical fiber for receiving laser energy. The tip may be coated with a high melting point material to absorb the radiation and heat the tip. The disclosures of each of the above patents, and all other patents cited in this specification, are incorporated herein by reference. U.S. Pat. No. 5,194,712 of Jones describes a single crystal diamond cutting tool with an anti-reflection coating bonded to the entry and exit faces of the cutting tool to provide efficient transfer of laser light, or to concentrate laser light at the desired incision.
Of the infrared lasers, which include CO.sub.2 (10.6 micron) and Nd:YAG (neodymium:yttrium-aluminum-garnet) (1.06 micron), the CO.sub.2 laser is most widely used for surgical applications of ablation and cutting of tissue. It is also more readily available and more economical, costing much less than other types of surgical lasers. While Ho:YAG and Nd:YAG lasers still emit light at a short enough wavelength that conventional optical delivery techniques can be used, because of its position in the far-infrared region of the electromagnetic spectrum, the CO.sub.2 laser cannot be delivered through quartz fiber optics, or silica or sapphire lenses, since these materials are opaque to the 10 micron wavelength and absorb the infrared laser radiation. (Materials that are commonly used with CO.sub.2 laser light, both as lenses and as mirrors, include sodium chloride, potassium chloride, zinc selenide, and germanium.) The CO.sub.2 laser light is typically directed through a series of mirrors in a complex articulating system through which the light is delivered to a handpiece containing a lens which will allow the beam to be focussed in a non-contact manner onto the target location. Examples of delivery optics for CO.sub.2 laser radiation are disclosed in U.S. Pat. No. 5,497,441 of Croitoru, et al., "Hollow Waveguide Tips for Controlling Beam divergence and Method of Making Such Tips"; No. 5,005,944 of Laakmann, et al., "Hollow Lightpipe and Lightpipe Tip Using a Low Refractive Index Inner Layer"; and No. 4,917,083 of Harrington, et al., "Delivery System for a Laser Medical System." Relatively recent developments in waveguide technology include a flexible hollow waveguide which is suitable for use with CO.sub.2 lasers having powers over 80 W. Such waveguides are disclosed in U.S. Pat. Nos. 5,440,664 and 5,567,471 of Harrington, et al.
It is known that single crystal type II diamond (pure carbon, effectively free of nitrogen impurity) has very low absorption at 10.6 microns, on the order of 0.03 cm.sup.-1, and also has high thermal conductivity, on the order of 2,000 W/m/K in comparison with other far-IR transmitting materials. High quality synthetic diamonds, including diamond films formed using chemical vapor deposition (CVD) have been made possessing similar mechanical, optical and thermal characteristics. For this reason laser cavity windows formed from diamond have been described for use in high power lasers, particularly CO.sub.2 lasers. See, e.g., U.S. Pat. No. 5,335,245 of Marie, et al.; and U.S. Pat. No. 5,245,189 of Satoh, et al. See, also, U.S. Pat. No. 5,194,712 of Jones, supra with regard to use of diamond for transmission of laser radiation, including that from a CO.sub.2 laser.
In recent years significant attention has been focused on the application of lasers to treating cardiovascular diseases, in particular, techniques for revascularization of ischemic myocardium. The procedure, laser transmyocardial revascularization (TMR), was first reported in the early 1980's following procedures which used a CO.sub.2 laser to form channels in damaged heart tissue to increase myocardial perfusion via the transport of oxygenated blood through the channels. (See, e.g., M. Mirhoseini, et al., "Myocardial Revascularization by Laser: A Clinical Report", Lasers in Surgery and Medicine 3:241-245 (1983).) This initial work was performed on an arrested heart using a low power (80 W) CO.sub.2 laser. Subsequent work in TMR led to the numerous laser systems which could be used on a beating heart, such as the one disclosed in U.S. Pat. No. 4,658,817 of Hardy ("Method and Apparatus for Transmyocardial Revascularization Using a Laser"), in which a CO.sub.2 laser was used. U.S. Pat. Nos. 5,380,316, and 5,554,152, of Aita, et al., assigned to CardioGenesys Corporation of Santa Clara, Calif., disclose the use of a CO.sub.2 laser or a Holmium:YAG laser for TMR procedures, however, the commercial system actually marketed by CardioGenesys is based upon a Ho:YAG laser with a fiber optic/lens contact-type delivery system. The wavelength emitted by the Ho:YAG laser, 2.1 microns, like the Nd:YAG, is sufficiently short to permit use of conventional optical delivery techniques, eliminating the delivery limitations experienced with CO.sub.2 lasers. U.S. Pat. No. 5,607,421 of Jeevanandam, et al., describes a laser TMR system which uses a thulium-holmium-chromium:YAG laser (THC:YAG) laser with conventional optical fiber delivery via a catheter passed through the left atrium.
Development of other laser TMR systems for investigational use has been reported by PLC Systems, Inc., of Franklin, Mass., Eclipse Surgical Technologies, Inc., of Sunnyvale, Calif., and Helionetics, Inc., of Van Nuys, Calif., all for use on a beating heart. The Eclipse TMR system uses a Ho:YAG laser with a fiber optic handpiece for contact delivery to the myocardium. The Helionetics system is based an excimer laser and uses conventional fiber optic delivery techniques. PLC Systems uses a high power (1000 Watt) CO.sub.2 laser in its Heart Laser.TM. with an articulated arm delivery system, such as that described in U.S. Pat. No. 5,558,668 of Lankford, et al., assigned to PLC Medical Systems, Inc.
Primary distinctions between the use of Ho:YAG or excimer lasers and CO.sub.2 lasers include that the CO.sub.2 lasers can create a transmural channel with a single pulse synchronized with the R wave (beginning of contraction) of a beating heart. (An exemplary synchronization system is disclosed in U.S. Pat. No. 5,125,926 of Rudko, et al.) The Ho:YAG and excimer lasers utilize low pulse energy and must fire multiple pulses over multiple cardiac cycles, typically without synchronization, in order to form a single channel. Another important distinction is in the delivery systems, with CO.sub.2 based systems using articulated arms, supplying the laser energy in a non-contact manner, thus requiring higher power laser sources and more invasive access methods, e.g., open chest surgery. Distinctions also lie in the relative costs and reliability of CO.sub.2 and excimer laser-based systems: CO.sub.2 lasers are relatively readily available, inexpensive and easily maintained, and many hospitals already possess or have access to such lasers. Excimer lasers are large, expensive, and difficult to maintain, requiring frequent service, and use highly toxic gas as the lasing medium.
The precision required for safe and controllable formation of multiple small diameter channels in the myocardium suggests that a contact or near-contact methods for application of laser energy would be preferred. Further, the ability to utilize contact delivery methods enables the use of less invasive procedures for obtaining access to the heart, e.g., small incisions between the ribs (thoracotomy) as opposed to open chest surgery. However, according to TMR techniques currently in use, the advantages of contact delivery must be offset by the lower ablative energy provided by shorter wavelength (mid- or near-IR) light.
In view of the above-identified deficiencies in the TMR prior art, there remains a need for a system and method for delivering laser radiation, especially radiation from a CO.sub.2 laser, in a precisely controlled manner as required for delicate surgical procedures. The delivery system and method disclosed in the following written description and drawings addresses and overcomes each of these deficiencies as well as providing other effective laser surgery techniques.